1. Field of the Invention
The present invention relates to a display panel in which a light emitting element formed on a substrate is sealed between the substrate and a cover member. In addition the present invention also relates to a display module in which an IC chip is mounted on the above-mentioned display panel. It should be noted that in the present specification, the display panel and the display module are collectively referred to as the light emitting device. The present invention further relates to an electronic apparatus that employs the above-mentioned light emitting device.
2. Description of the Related Art
Recently, techniques for forming a TFT on a substrate has been greatly advanced, and much developments have been made to apply those techniques to an active-matrix type display device. In particular, a TFT employing a poly-crystalline silicon film can operate at a higher speed since a field effect mobility (simply referred to as the mobility) thereof is larger than that of a TFT employing the conventional amorphous silicon film. Thus, it becomes possible to control pixels by means of a driver circuit formed on the same substrate as the pixels. Such the pixels was conventionally controlled by means of a driver circuit provided at the outside of the substrate.
The active-matrix type display device as mentioned above can exhibit various advantages such as a reduced fabricating cost, miniaturization of the display device, an increased fabricating yield, a reduced throughput or the like, by providing various circuits and elements on the identical substrate.
Furthermore, developments of an active-matrix type light emitting device having light emitting elements as a self-emission type element have been actively conducted. Such a light emitting device is also referred to as an Organic EL Display (OELD) or an Organic Light Emitting Diode (OLED).
Unlike a liquid crystal display, the light emitting device is of the self-emission type. The light emitting element has a structure in which a layer containing an organic compound (hereinafter referred to as the organic compound layer) that allows luminescence to be generated by applying an electrical field thereto is interposed between a pair of electrodes (an anode and a cathode). The organic compound layer usually has a layered structure. Typical examples therefor include a layered structure of “a hole transportation layer/a light emitting layer/an electron transportation layer” proposed by Tang et al. of Eastman Kodak Co. This structure has a high luminous efficiency, and most of light emitting devices about which research and development activities are currently being progressed employ this structure.
Although a luminescence in an organic compound includes a singlet excitation (fluorescence) and a triplet excitation (phosphorescence), the light emitting device of the present invention can use one or both of the above luminescence.
Alternatively, a layered structure in which a hole injection layer/a hole transportation layer/a light emitting layer/an electron transportation layer, or a hole injection layer/a hole transportation layer/a light emitting layer/an electron transportation layer/an electron injection layer are formed on an anode in these orders may be used. Furthermore, fluorescent dyes or the like may be doped into the light emitting layer.
In the present specification, all of the layers to be disposed between the cathode and the anode are collectively referred to as the organic compound layer. Accordingly, all of the above-mentioned layers such as the hole injection layer, the hole transportation layer, the light emitting layer, the electron transportation layer, the electron injection layer or the like are included in the organic compound layer.
A predetermined voltage is applied to the organic compound layer made of the above-mentioned structure via the pair of electrodes, and thus recombination of carriers occurs in the light emitting layer, thereby resulting in light emission. In the present specification, when the light emitting element emits light, the light emitting element is expressed as being driven. In addition, in the present specification, the light emitting element composed of an anode, an organic compound layer, and a cathode is referred to as the light emitting element.
Since the light emitting device is not required to employ a back light, a thickness and a weight of the display itself can be suppressed, as compared to a liquid crystal display. For that reason, the light emitting device has come to be used as a display section of a portable information terminal (a mobile computer, a portable telephone, a portable game apparatus, an electronic book or the like), instead of the liquid crystal display.
In order to suppress power consumption of the portable information terminal, it has been desired to suppress power consumption of the light emitting device to be employed as the display section.
Moreover, digitalization of a receiver or a VTR to be used in home as well as various equipment to be used in a TV or radio broadcast station has now proceeded. As the next step following digitalization of a broadcast system is digitalization of broadcast electric waves, i.e., realization of digital broadcasting. Vigorous research and development activities have been performed toward such a goal.
One scheme to be used for realizing a digital driving of the light emitting device is a temporal gradation display. More specifically, the temporal gradation display refers to a driving method in which the gradation display is performed by controlling a time period during which the light emitting element is to emit light in one frame period.
In the case where the light emitting device is digitally driven to perform the temporal gradation display, the required number of rewriting a digital video signal containing image information to be input to pixels is increased, as the number of gradation of an image to be displayed is increased. Consequently, power consumption of a group of driver circuits to be used for inputting the digital video signal into the pixels is increased, thereby resulting in increased power consumption of the light emitting device.
In addition, since the light emitting element is of the self-emission type, a time period during which the light emitting element is to emit light in one frame period varies depending on an image to be displayed. Accordingly, power consumption of the light emitting device in turn varies depending on an image to be displayed.
Furthermore, the magnitude of a current to flow through the light emitting element varies also depending on temperature. Even with a voltage of the same magnitude being applied across the electrodes of the light emitting element, the magnitude of a current to flow through the light emitting element varies due to temperature characteristics of the light emitting element. For example, as the temperature of the organic compound layer increases, a larger current tends to flow. Accordingly, as an environmental temperature under which the light emitting element is to be used increases, power consumption of the light emitting device increases, which in turn increases the brightness of the light emitting element.
In view of the above-described situations, the present invention is intended to suppress power consumption of a light emitting device and an electronic apparatus employing the light emitting device as its display section.